Aerosols aren't the only possible theory for an atmospheric sun shade. The most recent method for giving the Earth a little rest from the sun's rays, proposed by Roger Angel at the University of Arizona, involves sending trillions of lightweight, transparent discs out into space where they could block some incoming radiation.

The discs would act like small spaceships hovering about one million miles (1,609,344 kilometers) above the Earth's atmosphere in one big cloud. The long line of discs would have a diameter of about half the Earth's (which is about 7,900 miles, or 12,700 kilometers) and be 10 times longer. Angel designed the lightweight flyers to be made of a transparent film, measure about 23.6 inches (60 centimeters) in diameter and covered with tiny holes.

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The end result would be that the discs could direct as much as 10 percent of the sun's light passing through the cloud away from the Earth. This could reduce sunlight by 2 percent over the surface of the entire planet, cooling things down significantly. The lighter weight of the discs provides an advantage over the previously-mentioned glass shield theory because getting them outside the Earth's atmosphere wouldn't require quite as much effort. In fact, hydroelectric power could be used to power electromagnetic launchers, which Angel suggests firing every five minutes for a duration of 10 years in order to send off an acceptable number of flyers. The price tag for the project is anything but tiny -- the sunshade flyers might cost several trillion dollars, or about $100 billion every year, but Angel believes the benefits of his sun shade system would last for about 50 years.

Although scientists have been speculating for decades about how an atmospheric sun shade could work, many skeptics have pointed out that the scheme won't work at all or at least that it won't help other aspects of power generation. The unfortunate side effect of shading the Earth is the unintentional reduction of peak solar power productivity. An atmospheric sun shade, whether it involves scattering particles into the air or sending out millions of thin flying discs, could deflect as much as 20 percent of solar power from power producing plants.

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